Macbeth does not fear punishment after death. He hesitates before murdering Duncan because he is afraid that someone will find out that he has committed a crime and fears he will be punished in this life for his crimes. More importantly, Macbeth fears that he will be inadequate as the new king. He knows that everyone in Scotland loves Duncan because he is a good, just king, and Macbeth is not sure that he will be able to match Duncan and develop a rapport with the people. After Macbeth murders Duncan, he is so afraid that someone has heard him commit the crime that he runs to see his wife still holding the bloody knife. Then much later in the play, Macbeth realizes that he is all alone in his venture and that no one in his court loves him the way they loved Duncan. Macbeth wants to be a great, powerful king, so he fears anything that threatens his position.
If you read Macbeth's soliloquy in the beginning of act 1 scene 7, you will know that while examining the pros and cons of Duncan's murder, Macbeth was more seriously concerned about the punishment he would have to receive in this world itself, rather than in the world to come after death. Macbeth was very much conscious of the retributive justice in this world and, had it been a question of punishment after death, he would have easily and quickly killed Duncan.
Yes, Malcolm knows Macbeth better than his father, King Duncan. Duncan was overwhelmed by his general Macbeth's valorous loyalty, and was all praise for him in whom he could put absolute trust. On the other hand, Malcolm saw how the fair Macbeth turned foul, killed his good old king, usurped the throne, and unleashed a reign of terror. Having taken an asylum in the English King's court, having received the details of Macbeth's bloody tyranny, Malcolm came to realize that Macbeth was a "(dead) butcher".